Oh! that's a lot of questions!
It depends on YOU. Anyhow in the US there are very few one year programs, so it wouldn't matter too much. There are many considerations but basically the advantage of a two year program is its length which allows you to dedicate more time, and build closer relationships as well as do an internship. The advantage of one year programs is that it costs less and you get out faster. But there are a lot of differences on a program by program basis, and there's no substitute for individual research.
As for double degrees, again this depends more on what you want. If you think you are dedicated enough to deal with the workload and want the degree, I would say "why not"??? After all, you don't pay much more and you do come away with a second degree.
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